yep, firehose is the go to standard on fire crews. although getting ahold of some in the right diameter if you're not on a crew might be tough. do they sell cloth garden hose?

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Kydex?I might consider 1000D Cordura, two or three layers if sewn in a sheath, or, since you are talking fabric, use it to wrap the file and tie it like a parcel rather than sew a tube. I sewed a tube to carry a 36" bow saw blade from 100D Cordura. I don't expect it will last forever, but it was used to fly the blade in and out, not keep it masked day in day out, so it will do three or four trips. That saw is much harder on the material than a file will be.Wrapping will be easier on the fabric than making a tube sheath.Hypalon would be good. I might be tempted to sew that like a long, shallow envelope, with a flap closure, so that the file is dropped in on a long side, rather than slid in from a short side, if you follow. Again, that saves a lot of rubbing. Actually, could do that with the Cordura too.Anything that is sufficiently waterproof to keep water away, will hold any water that gets in there. At least with the synthetics though they don't tend to just hold dampness in their fibres the way leather can do.

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